JPA Spring Data entity to be used outside of transaction

Sergei Ledvanov :

I have a Spring Boot application with a service that returns a Spring Data entity that is exposed to a controller. The problem is that I know it's not a good idea to use entities outside of DB transactions, so what would be the best practices?

Consider the following service:

@Transactional
public MyData getMyData(Long id) {
    return myDataRepository.findById(id);
}

where MyData is a database @Entity and myDataRepository is a JpaRepository

This service method is called from a controller class, that sends this object in JSON format to a client that calls this method.

@RequestMapping("/")
public ResponseEntity<?> getMyData(@RequestParam Long id) {
    return myService.getMyData(id);
}

If I expose MyData to a controller, then it will be exposed outside of a transaction and might cause all kind of hibernate errors. What are the best practices for these scenarios? Should I convert entity to POJO in side the service and return MyDataPOJO instead of MyData in MyService?

Viking :

Using entities outside of transactions does not necessarily lead to problems; it may actually have valid use cases. However, there's quite a few variables at play and once you let them out of your sight things may and will go south. Consider the following scenarios:

1. Your entity doesn't have any relationships to other entities or those relationships are pretty shallow and eagerly fetched. You retrieve that entity from repository, detach it from persistence unit (implicitly or explicitly) and pass to controller. Controller does not attempt to modify the entity; it only serializes it into JSON - totally safe.
2. Same as above but controller modifies the entity before serializing it into JSON - again, totally safe (just don't expect those changes to be reflected in DB)
3. Same as above, but you've forgotten to detach the entity from PU - ouch, if controller changes the entity you may either see it reflected in DB or get transaction closed exception; both most likely being unintended consequences.
4. Same as above, but some of entity's relationships are lazy. Again, you may or may not get any exceptions depending on whether these lazy properties are being accessed or not.
5. And there are so many more combinations of intentional and unintentional design choices...

As you may see, things can get out of control very quickly. Especially so when your model has to evolve: before long you're going to find yourself fiddling with JSON views, @JsonIgnore, entity projections and so on. Thus the rule of thumb: although it may seem tempting to cut some corners and expose your entities to external layers, it's rarely a good idea. Properly designed solution always has a clear separation of concerns between layers:

• Persistence layer never exposes more methods or entities than required by business logic. More over, the same table(s) can and should be mapped into several different entities depending on the use cases they participate in.
• Business logic layer (btw this is your API, not the REST services! see below) never leaks any details from persistence layer. Its methods clearly define use cases from the problem domain.
• Presentation layer only translates API provided by business logic into one or another form suitable for client and never implements additional use cases. Keep in mind that REST controllers, SOAP services etc logically are all part of presentation layer, not business logic.

So yeah, the short answer is: persistence entities should not be exposed to external layers. One common technique is to use DTOs instead; besides, DTO objects provide additional abstraction layer in case you need to change your entities but leave API intact or vice versa. If at some point your DTOs happen to closely resemble your entities, there are Java bean mapping frameworks like Dozer, Orika, MapStruct, JMapper, ModelMapper etc that help to eliminate the boilerplate code.

Try googling "hexagonal architecture". This is a very interesting concept for designing cleanly separated layers. Here's one of the articles on this subject https://blog.octo.com/en/hexagonal-architecture-three-principles-and-an-implementation-example/; it uses C# examples but they're pretty simple.